before i build this (i'm doing something more like Mince Wobley's idea), it looks like there's going to inevitably be a knob on the end. would this be easier or harder than if the entire thing was the same thickness?
it doesn't matter as long as you don't have a hard edge at the knob. the main hose can be thin if the circumference of the knob graduadually decreases to the circumference of the hose (in layman's terms it needs to slope like a funnel). you can do this with duct tape or latex.
also you need to make sure to give yourself a few enemata beforehand, and you shouldn't consume anything but water.